In water-cooled reactors, activated corrosion and wear products are dissolved in the water. The solubility of these products varies with reactor conditions, i.e. mainly temperature, pH at a given dissolved H2 concentration, and chemistry of the primary coolant, all of which may change along the coolant circuit and during the fuel cycle. These variations cause metallic material dissolution at one point of the primary coolant circuit and deposition onto the circuit surfaces at another. The deposits are generally generated by precipitation reactions and are usually made of solid particles, agglomerated together that can be strongly attached to the underlying substrate. The term CRUD, an acronym for Chalk River Unidentified Deposits, is commonly used to characterize these deposits. CRUD deposition is temperature dependent and tends to deposit more rapidly at the highest temperature surfaces further exacerbating the condition. In the nuclear reactor circuit, the highest temperature surfaces are the nuclear fuel rod claddings. As thermal and hydraulic conditions play a major role in the deposition, within the reactor core deposits can be expected at sub-cooled nucleate boiling regions, this mechanism favoring the impurities concentration and the solubility decrease, at higher temperature surface locations, at low turbulence regions, and at high impurity concentration locations in the primary coolant. As time passes, materials can collect on an exterior oxidized surface of the zirconium base alloy claddings of the nuclear fuel rods that contain the fissile material such as enriched uranium dioxide or mixed oxide made of uranium dioxide and plutonium dioxide. CRUD buildup on fuel rod claddings reduces the heat transfer coefficient from the fuel rod surface to the primary coolant resulting in higher fuel rod temperature and as such in increased oxidation of the zirconium cladding. Thicker local CRUD allows specie precipitation, such as boron, which, if exceeding a given quantity per unit area of fuel rod, could lead to local nuclear power shifts.
To perform analysis of CRUD deposited on the fuel assemblies, samples must be taken by mechanically brushing or scraping the exterior of the fuel rods. The systems used to perform this mechanical brushing consist of a mechanical arm having at one end a rotating brush of a given configuration and bristle material. The mechanical brush brushes the surface of the CRUD, collecting, depending on the pressure applied to the arm and the selection of bristle materials, part or all of the CRUD deposit. The systems to perform the mechanical scraping include a rigid member that the fuel rod is pressed against, thereby shearing the loose CRUD from the rest of the nuclear fuel rod when the rigid member is moved over the fuel rod. Another method used to collect CRUD deposits is the rotating wheel scraping method where a highly abrasive material wheel is pressed while rotating against the surface of the fuel rod. All of the above mechanisms are embodied in a manual or mechanical device.
More specifically, CRUD is a non-homogeneous material and porosity changes in the thickness of the deposit. Usually the small thickness deposits and the exterior of the higher thickness deposits are characterized by a relatively high porosity in the order of 70% to 80%, and higher, having a fluffy friable aspect. In case of thick deposition, up to 60% of the deposit thickness may be tenacious with porosity less than 50%. The CRUD composition can highly affect the fuel rod cladding corrosion locally, either by acting as a thermal insulator or by chemically favoring the corrosion process. The knowledge of elemental distributions at various locations of the cross-section of the CRUD (Fe, Cr, Ni, Co, Si, Zn, etc.), of the individual crystal chemistry, of the size and morphology of the crystals is of fundamental importance to define a suitable surveillance scheme. As such a method is needed that allows a differentiate collection of CRUD, better responding to the porosity and density characteristics of the deposition and also providing better answers on what quantities and what type of CRUD material can be entrained during start-up or during sequence exchange (fluffy CRUD) and what quantity and type of CRUD remains attached to the pin surface irregardless of the operating conditions.
Existing devices have a major drawback that limit the effectiveness of the CRUD analysis: fluffy and tenacious CRUD are collected together leading to mixing of samples and losing important information needed to define the best surveillance scheme and associated action plan for the further operation of the nuclear reactor.
U.S. Pat. No. 7,132,651, incorporated herein by reference, teaches a method for collection and analysis of CRUD flakes.